General Election 2015: The leaders' TV debate Achilles heels

No-one's perfect - and all seven of the party leaders in this week's TV debate are no exception. These are the chinks in their armour.

David Cameron: A tense prime minister

After doing his best to avoid the debates altogether, the Prime Minister must work hard to convince viewers he really wants to be there. If it looks like he's there under duress, it's all anyone will be talking about.

Embracing the debates means going in with a positive attitude. But this is a problem because his big weakness in Downing Street has been developing a very tense style that is so severe it actually restricts him from smiling.

Cameron is also under pressure to appear more prime ministerial than the others.

It's going to be a tricky dilemma to resolve: combining being positive with staying above the fray might just prove too much for his inflexible communication approach.

At least he gets the last word.

Ed Miliband: Playing it safe

For a politician who's shown he's just not likeable enough to be a real asset to his party, Ed Miliband should view the TV debates as a big opportunity to turn things around.

He needs a big moment when he speaks on behalf of the British people, preferably with a good dose of humour, and gets voters thinking he's someone to laugh with - and not laugh at.

Miliband will be under intense pressure to deliver a game-changing performance, so the worst thing he can do is play it safe.

With the stakes so high the Labour leader is likely to be more focused on getting his central messages out than on delivering a moment of brilliant rhetoric which goes beyond the usual 'I've heard this all before' fare.

Nick Clegg: Past his sell-by date

Defending the government never works out well. Five years after being catapulted to power off the back of the TV debates, 'Cleggmania' seems like a distant memory.

Instead, the Lib Dem leader now brings a lot of baggage to his podium. His claim that the Liberal Democrats will champion both fairness and economic rigour is a nuanced and sophisticated strategy that simply doesn't work in the debate format.

Being balanced might keep the party faithful happy, but TV viewers are a different matter. As soon as Clegg starts getting into detail he may lose half the people he needs to win over.

So he needs to show he's matured without getting boring - a tough ask for someone essentially on the defensive throughout this campaign.

Nigel Farage: Strong expectations, weak policies

Leading a party like Ukip has its drawbacks and it's not just the members.

It's also the right-wing policy platform they support which is so easy for opponents to attack. Tolerant voters are suspicious of Ukip's views, so Farage must do his best to avoid talking too much about what he actually believes in.

The Ukip leader has another problem, too.

After appearing as the insurgent for so long, Farage is expected to not just win but turn over the applecart while doing so. That means he stands to lose more than he gains because if he gets lost among everyone else it'll be a disaster.

Natalie Bennett: No confidence

The Greens' leader has so far shown that she just doesn't have the interpersonal skills to really get her message across.

She probably lacks the confidence to come up with a game-changing moment which changes the conversation and persuades voters to give her a second look.

It's a problem of visible passion. Lots of people care about her party's two big issues, the environment and income inequality. So why is their leader so academic and detached when talking about them?

Voters need warmth and passion, and find it lacking in Natalie Bennett. On the upside, she's messed up so often the expectations are very low.

Nicola Sturgeon: Short shrift

Nicola Sturgeon is set to play a big role in any hung parliament negotiations so will be looking to put in a professional, confident performance in the TV debates. She needn't rock the boat.

On the other hand, she needs to make a good fist of attacking Ed Miliband and asking him who understands Scotland's needs better.

If she seems to be off the boil, fluffing her lines and flopping with viewers, Labour can simply say she hasn't got the stature of a first-rate politician.

Leanne Wood: What am I doing here?

The Plaid Cymru leader should count herself lucky: she's appearing on a national TV debate although her party is only likely to get a handful of MPs. Her big test is to persuade non-Welsh voters that she actually matters.

In her big speeches and interviews she comes across well although her delivery is laboured and her language can sometimes be convoluted. She is an appealing politician with an opportunity to boost name recognition.

She comes across as different from the three main party leaders, too, and not just because she's a woman: her accent obviously helps rather than hinders her cause. The question she has to prevent viewers asking is: What difference will she make to how Britain is governed?